| Re: frequency of crossovers at meiosis |
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Group: sci.bio.evolution · Group Profile
Author: dhoytdhoyt Date: Feb 24, 2008 11:09
> I read that mendels second law (saying that traits are inherited
> independently) if either
>
> 1) the genes regultating the traits reside on different chromosones
> 2) the genes are far from each other on the same chromosone
>
> Can this really be true for 2) ?
> If even nr of crossovers (including 0) is as common as odd, the traits
> are more likely to follow each other than if the genes are at separate
> chromosones.
>
> Am I thinking about this wrong ?
Maybe this will help clear up your confusion.
Consider two loci (genes) that are on the same chromosome but far
enough apart so that there is ALWAYS at least a single crossover
between them. Since a crossover involves only a pair of chromatids,
there will be a pair of non-recombinant chromatids for every pair of
recombinant chromatids. (This just says that AB, ab, Ab, and aB
gametes will be equally frequent.) Therefore the %%recombination
between the A and B loci will be 50%% when there is always one
crossover between them.
But what if there are some meioses with a double crossover? These will
also produce the 4 gametic types in equal frequencies. Why? Because
there are three types of double crossovers: two-strand doubles, three-
strand doubles, and four-strand doubles. If you can find an older
genetic text it will show you the genetic consequences of each type of
double exchange. It turns out that the 4 gametic types, AB, ab, Ab,
and aB are produced in equal frequencies, just like in the single
exchange.
So, if two loci are far enough apart that there is a least one
crossover between them in 100%% of the meioses they will show 50%%
recombination, even if there are also some meioses that have a double
exchange.
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